The manufacture of tools, e.g. cutting blades for milling or drilling, is frequently performed by a sintering process. Before undergoing sintering, compacts are formed from metallic powder. An eccentric press or hydraulic press is used to press the metallic powder into shape. The invention relates to a powder hydraulic press.
The upper and lower rams are operated by means of appropriate press cylinders in a powder hydraulic press. It is not only necessary to apply a considerable pressure by means of the press cylinders, but the rams also require to be moved to a predetermined position each in order to avoid damage, on one hand, and ensure desired reproducible dimensions for the compact, on the other.
Powder presses of this type are known to measure both the length passed through by the rams and the force which is applied. This way makes it possible to ensure a desired compaction with dimensions being predetermined. To measure forces, it is known to arrange a load cell between the press cylinder and the ram.
Once the rams have taken their predetermined positions and the maximum compression force is reached the press cylinders undergo a reversal of their motion and it is specifically the upper ram which is moved out of the die-bore to enable the compact to be ejected by means of the lower ram.
As is further known the sudden relief of the compression power is disadvantageous. The compact exhibits a certain spring-back behaviour and there is a risk of undesirable cracks forming in the compact during the spring-back, which have an adverse effect in sintering and cause defective compacts. When the compression power is removed it is known to maintain a so-called live load, e.g. by means of a spring. A spring, however, has only one predetermined characteristic line.
During pressure relief, it is also imaginable to jointly displace the lower and upper rams in a way to produce a certain live load which gradually decreases. The existing hydraulic press cylinders, however, make it nearly impossible to obtain a harmonized motion of the upper and lower rams.
It is the object of the invention to provide a hydraulic press for compressing metallic powder or the like to form compacts wherein a controlled spring-back of the compact becomes possible.